1. Field of the Invention
This invention relates to a position detecting apparatus for detecting the position of an object, and particularly to the construction of a position detecting apparatus using a magneto-resistance effect (MR) element which is suitable for position detection with high resolution.
2. Description of Related Art
A position detection with high resolution is required for a zoom lens or focus lens equipped in a video camera, for example.
A lens-barrel of the video camera is constructed by plural lens systems containing a zoom lens and a focus lens to perform a zoom function and a focus function. For example, four groups of lens systems are provided to the lens-barrel. The lens systems of first and third groups are fixedly disposed while the zoom lens system serving as a second group is disposed movably in an optical axis direction, and the focus lens system for focusing, compensation and imaging operations are likewise disposed movably in the optical axis direction. The zoom lens is driven by a stepping motor for example, and the focus lens is driven by a voice coil motor.
In such a video camera, a position control is carried out such that the position of the focus lens is varied in correspondence to the movement of the zoom lens so that an imaging position is fixed at all times. Particularly in "Manual Zoom Tracking" mode that the zoom lens is manually moved, the focus lens must be moved in such a manner as to trace along a predetermined focus, so that the position control of the focus lens is required to be carried out with precision of about 20 .mu.m. In order to satisfy such a requirement, this applicant has proposed an MR type position detecting apparatus capable of carrying out position detection with high precision.
The MR type position detecting apparatus comprises a magnet having polarity which is alternately magnetized at a minute interval of 150 to 400 .mu.m , and an MR element whose resistance value is varied in response to the magnetic field of the magnet, and a higher-precise position detection signal can be obtained by finding a magnetization pitch pattern of the magnet or the MR element.
In the MR type position detecting apparatus as described above, the magnet is usually secured to a movable body, for example, a lens holder for supporting the focus lens, and the MR element is fixed to a fixed side, for example, a predetermined portion of the lens-barrel. The magnet and the MR element are disposed so as to be confronted to each other at a predetermined gap interval when the movable body is located at a predetermined position within a movable range.
Here, it is preferable to accurately adjust the gap interval because the gap interval affects the output voltage of the MR element. For example, for the magnet having a magnetization pitch of 264 .mu.m, it is apparent from FIG. 16 that the gap interval must be set to 0.08 to 0.18 mm to obtain an output above 40 mV from the MR element.
However, it has been substantially impossible to keep the gap interval between the confronting magnet and MR element to a predetermined value because of accumulated tolerance of respective parts which are used in the MR element.
Therefore, the MR type position detecting apparatus as described above has the following problems. A complicated gap adjustment mechanism is further required, and thus the construction is more complicated. In addition, a gap adjustment work is required for each set after a fabrication process of desired parts, and thus a manufacturing process is ineffective.